Definition and handling of constants TYPO3_MOD_PATH and PATH_typo3_mod have been dropped.
These constants were important for modules that were still not called through the mod.php_DISPATCH system that was introduced in TYPO3 CMS version 4.2.
It is required to route modules through typo3/mod.php from now on in case the module relies
on the definition of those constants.

The following old entry scripts were removed:

typo3/sysext/cms/layout/db_layout.php, use \TYPO3\CMS\Backend\Utility\BackendUtility::getModuleUrl('web_layout') to link to the module

typo3/sysext/cms/layout/db_new_content_el.php, use \TYPO3\CMS\Backend\Utility\BackendUtility::getModuleUrl('new_content_element') to link to the module

Path resolving may fail for scripts that rely on entry points different from typo3/mod.php
Constants TYPO3_MOD_PATH and PATH_typo3_mod are not defined anymore and scripts may
throw a PHP warning if they try to access these constants.

Installations may fail if linking to modules that use an entry script with a conf.php file that do not
use $MCONF['script']='_DISPATCH';. Those modules must be adapted to the mod.php entry point and may
need adaption of further references that are defined relative to the entry script.

Searching for extensions with backend modules that define TYPO3_MOD_PATH is a good start to find unwanted usages.